UPDATE 2: Got it working. *facepalm*

Of course I had to do the command:

sudo cat /opt/retropie/configs/all/retroarch-joypads/XboxGamepad(userspacedriver\).cfg >> /opt/retropie/configs/all/retroarch.cfg

… to get the controller working in the emulators, with input_hotkey_enable_btn and all those features.

I don’t know what’s up with the latest RetroPie 2.6 revision, before I started messing around with “install binary scripts” and stuff my old USB-joypad .cfg-file was defined and ran from the folder:

/opt/retropie/emulators/retroarch/configs/

(Never needed to touch the retroarch.cfg file for setting up my old controller, in other words)

Anyway, problem SOLVED :D

/Alucard